This invention is directed to methods for preparing linear or straight-chain, and cyclic polysiloxanes.
Acid equilibration (polgerization) of siloxanes such as tetramethyl cyclotetrasiloxane (D.sup.H 4) and pentamethyl cyclopentasiloxane (D.sup.H 5) has been performed in the past, but the method of our invention relates to the polymerization of highly pure silanol-free hexamethyldisiloxane and highly pure mixtures of silanol-free D.sup.H 4 and D.sup.H 5 containing no more than about 100 parts per million (ppm) water, with anhydrous trifluoromethane sulfonic acid (triflic acid, CF.sub.3 SO.sub.3 H), at room temperature, to provide access to linear or straight-chain methylhydrogen siloxanes of molecular weight up to about MD.sup.H.sub.2200 M.
Hydrosilation (.ident.SiH+CH.sub.2 .ident.CHR=SiCH.sub.2 CH.sub.2 R) of alpha-olefins with methylhydrogen siloxanes has been performed in the past, but the method of our invention relates to hydrosilation of alpha-olefins with high molecular weight methylhydrogen siloxanes (degree of polymerization up to about 2200), efficiently performed to provide alkylmethyl polysiloxanes with low .ident.SiH content.
Cyclosiloxanes have been alkylated in the past but afford products with substantial levels of residual refill. The method of our invention relates to the use of highly pure silanol-free tetramethylcyclotetrasiloxane (CH.sub.3 HSiO).sub.4 and highly pure silanol-free pentamethylcyclopentasiloxane (CH.sub.3 HSiO).sub.5 containing no more than about 100 ppm water, that are 100% alkylated, leaving no detectable residual .ident.SiH, using anhydrous alpha-olefins ranging from ethylene gas to C30+ alkenes in the presence of a dried anhydrous platinum supported on carbon catalyst.
It is known that methylhydrogen chlorosilanes can be hydrolyzed and condensed to obtain silicone fluids retaining a high proportion of reactive hydrogen, but typically they contain 1% or more of branch sites. The reactions are often difficult to control, and the results are erratic (i) sometimes producing useless gels or hard brittle solids instead of fluids, and (ii) hydroxyl substitution on silicon (.ident.SiOH) which leads to gelation. This limits the usefulness of methylhydrogen siloxanes as starting materials for polymer synthesis.
These disadvantages have been overcome in our invention by utilizing highly pure essentially anhydrous silanol-free organodisiloxanes, and highly pure essentially anhydrous silanol-free methylhydrogen cyclic siloxanes (D.sup.H 4) and (D.sup.H 5), as starting materials.
Since our starting materials contain no water or residual .ident.SiOH, branching and cross-linking do not occur. Therefore, our end products are not branched-chain polysiloxanes or gels, but linear or straight-chain polysiloxanes. The interested reader is referred to U.S. Pat. No. 2,469,890 (May 10, 1949) for illustrations showing the structural differences between linear or straight-chain and branched-chain polysiloxanes.
Our invention, therefore, is in sharp contrast to the gel containing cross-linked oily materials taught in U.S. Pat. No. 2,491,843 (Dec. 20, 1949); or the cross-linked high polymers bearing SiOH end groups, described in the European Polymer Journal, Vol. 29, No. 1, Pages 15-22, (1993). Furthermore, our alkylated products are prepared from highly pure essentially anhydrous silanol-free starting materials and contain low or no residual .ident.SiH.